Rep. Jerrold Nadler was not the champion the new left wanted, but he quickly proved to be the one it needed in the hunt to impeach President Trump.

As chairman of the House Judiciary Committee, Mr. Nadler has launched a sweeping corruption investigation of the president, spearheaded demands to see the full Mueller report and seems unafraid to explore impeachment.

The vigorous pursuit of Mr. Trump impressed the new breed of far-left lawmakers and activists who have taken hold of the Democratic Party. They often eye with skepticism the old guard leaders such as Mr. Nadler, a long-serving New York liberal whose district includes Wall Street.

“It seems like he is the right person at the right time — a sturdy hand on the rudder,” said Adam Green, co-founder of the Progressive Change Campaign Committee, which leads the charge of what it calls the party’s “Elizabeth Warren wing.”

Other old-school liberals such as House Speaker Nancy Pelosi of California and House Ways and Means Committee Chairman Richard E. Neal of Massachusetts disappointed the party’s new generation by not aggressively pushing its far-left agenda.

“Many of the fights within the Democratic Party right now are not just relegated to progressive Democrats versus conservative Democrats, but in many cases it is the more senior, long-serving members versus those with a more immediate finger on the pulse of where the public is on issues like the Green New Deal or ‘Medicare for All,’” Mr. Green said.

“Among the longer-serving members, Jerry Nadler is more in touch and willing to be bolder than some others,” he added.

On Monday, Mr. Nadler subpoenaed former White House counsel Don McGahn, saying he is a “critical witness” on whether Mr. Trump obstructed justice. He also has issued a subpoena demanding the unredacted 448-page report by special counsel Robert Mueller and the “underlying evidence” he collected investigating suspected collusion between Mr. Trump’s campaign and Russian officials.

He said he saw plenty of evidence of obstruction of justice, which is the Democrats’ chief argument for impeachment after the report cleared Mr. Trump of conspiring with the Kremlin.

“Obstruction of justice, if proven, would be impeachable,” he said on NBC’s “Meet the Press.”

Mr. Nadler, 71, has been in politics his entire adult life. He has spent the past quarter century in Congress representing his Manhattan district.

Though a dyed-in-the-wool traditional liberal, he always has seemed to know where his party is moving and kept pace with it.

When the Occupy Wall Street protest began in 2011 with the takeover of New York’s Zuccotti Park, Mr. Nadler sided with the ragtag demonstrators and against his infuriated constituents in the financial district.

“The main thing is what’s going on. This is a national issue, not a neighborhood issue,” he told The Washington Times back then.

When President Obama needed support for the Iran nuclear deal, Mr. Nadler was the only Jewish member of the New York delegation to endorse the deal, which critics said would doom Israel.

His support was considered pivotal, and Mr. Obama personally lobbied him. Despite severe criticism, he went on to garner 89% of the vote over a primary challenger who made the Iran nuclear deal a top issue.

Mr. Nadler received his liberal pedigree at an early age when his father’s New Jersey chicken farm failed.

“I started becoming politicized when I was growing up because there were two names that were never pronounced except with disdain,” Mr. Nadler told The New York Times in 1992 when he was elected to the House. “One was Dwight D. Eisenhower, and the other was Ezra Taft Benson, his secretary of agriculture. I didn’t know what they did, but whatever they did made it impossible for chicken farmers to produce eggs without losing money.”

He entered the political fray in 1977, serving as a member of the New York General Assembly while going to law school at night. A year later, he earned his law degree.

As a New York politico, Mr. Nadler had run-ins with Mr. Trump. In the 1980s, when Mr. Nadler was an assemblyman, he opposed Mr. Trump’s plan to build the world’s tallest skyscraper on the Upper West Side. Mr. Trump nicknamed him “Fat Jerry.”

Mr. Nadler, who is 5 feet, 4 inches tall, struggled with obesity for most of his life. By the early 2000s, he had reached 338 pounds and was unable to climb a single flight of stairs at the Capitol.

After undergoing stomach-reduction surgery at Mount Sinai Hospital in Manhattan during Congress’ summer recess in 2002, Mr. Nadler eventually cut his weight in half and has kept it off.

A skilled lawyer and politician, Mr. Nadler emerged as a formidable opponent for Mr. Trump. The chairman is regarded as someone who knows not only the immense reach but also the precise limits of congressional authority to extract information from the executive branch.

“He is especially suited to think through serious problems and not make false steps. This is a very thoughtful, deliberate man,” said Hank Sheinkopf, a Democratic consultant in New York who has known Mr. Nadler for decades.

“Whatever you see is not off the cuff. Whatever you see is very thoughtful and measured,” he said. “And he has a history of not liking Donald Trump, as does anybody who was an elected official on the Upper West Side in the years when Trump was building his massive project over the rail yards.”

The chairman also is not new to impeachment.

Mr. Nadler watched with disdain as the Republican-led House impeached President Clinton.

“This is a partisan coup d’etat that will go down in infamy in the history of our nation,” he said in the House debate of the articles of impeachment in 1998. “Mr. Speaker, this is clearly a partisan railroad job.”

Back then, Mr. Nadler also argued against the release of the full report by independent counsel Kenneth W. Starr’s investigation of Mr. Clinton. He protested revelation of grand jury testimony, including salacious material, that he said may not be true.

The full Starr report was made public.

Mr. Nadler now is arguing on the other side of the issue for the report by Mr. Mueller, though changes in the law have made it unlikely that an unredacted version will be released.

In 2006, Mr. Nadler urged his Justice Committee colleagues to explore impeachment of President George W. Bush over his authorization of domestic surveillance without court review.

The issue was on a long list of grievances that Democrats said warranted impeachment. But the congressman ultimately backed Democratic leaders who decided in 2007 that it was too close to the election and the end of Mr. Bush’s final term to bring up articles of impeachment.

This time, Mr. Nadler said he won’t leave a stone unturned in search of evidence of wrongdoing by Mr. Trump.

He will do it by the book, methodically and with unrelenting determination, according to those who know him.